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Contents :

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INTRODUCTION:
A Polymer is a substance composed of molecules with large molecular mass composed of repeating structural units or monomers, connected by covalent chemical bonds.
The word is derived from the Greek, polu, “many”; and meros, “part” . Well known example of polymers include plastics, DNA, Proteins.
Natural polymer materials such as shellac and amber have been in use for centuries. Biopolymers such as proteins (for example hair, skin and part of bone structure) and nucleic acids play crucial roles in biological processes. A variety of natural polymers exists, such as cellulose, which is a main constituent of wood and paper.

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Significant advances have been made in the development of various drug delivery devices with the help of polymers.
They have better physical, chemical& biological properties for efficient therapy. 6

DEFINITION :

DEFINITION Polymers are defined as very large macromolecules consisting of repeating units of monomers.
The monomers can be linked together to generate a linear polymer.
Two types of polymers are there:
1 )Linear&Branched polymers .
2) Cross linked polymers. 7

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The word, polymer, implies that polymers are constructed from pieces (monomers) that can be easily connected into long chains (polymer). Many + Parts This name hints at how polymers are made Polymer Polymer Monomers

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Cotton: a natural polymer What is its building block (monomer)?

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Cotton fiber is mostly cellulose, and cellulose is made of chains of the sugar, glucose linked together a certain way.

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Synthetic polymers :

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POLYMER STRUCTURE AND PROPERTIES:
STRUCTURE:
The structure properties of a polymer relate to the physical arrangement of monomers along the backbone of the chain. It has a strong influence on the other properties of the polymer. For example a linear chain polymer may be soluble or insoluble in water depending on whether it is composed of polar monomers such as ethylene oxide or non polar monomers such as styrene.
On the other hand 2 samples of natural rubber may exhibit different durability even though their molecules comprise the same monomers.

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The terms configuration and conformation are used to describe the geometric structure of a polymer.
Configuration refers to the order that is determined by chemical bond. Configuration of a polymer can not be altered unless chemical bonds are broken or reformed.
Conformation refers to order that arises from the rotation of molecules about the single bonds.
CONFIGURATION : 2 types of polymer configurations are cis and trans. 3 distinct structures are there. They are Isotactic
Syndiotactic
Atactic H H -CH2 H
C = C C = C
-CH2 CH2- H CH2

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Isotactic Syndiotactic CONFORMATION: If two atoms are joined by a single bond then rotation about the bond is possible since, unlike a double bond, it doesn’t require breaking the bond.

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The ability of an atom to rotate this way relative to the atoms which it joins is known as an adjustment of the torsional angel. If two atoms have other atoms or groups attached to them then configurations which vary in torsional angle are known as conformations. several possible conformations are anti (trans), eclipsed (cis) and gauche (+ or -).
A branched polymer is formed when there are side chains attached to a main chain. Polymers that incorporate more than one kind of monomer into their chain are called copolymers. There are 6 types of copolymers.
Random copolymer
Block copolymer
Graft copolymer
Alternating copolymers
Periodic copolymers
Statistical copolymers.

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MONOMER IDENTITY :
Polymer nomenclature is generally based upon the type of monomers comprising the polymer. Polymers that contain only a single type of monomer are known as Homopolymers. Example Poly(styrene) Polymers containing a mixture of monomers are known as Copolymers. Example Ethylene-vinyl acetate.
A polymer molecule containing ionizable subunits is known as a Polyelectrolyte. An Ionomer is a subclass of polyelectrolyte with a low fraction of ionizable sub units.
CHAIN LINEARITY :
The simplest form of polymer molecule is a straight chain or linear polymer, composed of a single main chain. A branched polymer molecule is composed of a main chain with one or more substituent side chains or branches. Types of branched polymers include star polymers, comb polymers and brush polymers.

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If the polymer contains a side chain that has a different composition or configuration than the main chain the polymer is called a graft polymer. A cross link suggests a branch point from which four or more distinct chains emanate. A polymer molecule with a high degree of cross linking is referred to as a polymer network. Sufficiently high cross link concentrations may lead to the formation of a infinite network also known as a gel.
CHAIN SIZE:
Polymer bulk properties may be strongly dependent on the size of the polymer chain. Like any molecule, a polymer molecule size may be described in terms of molecular weight or mass. In polymers, however the molecular mass may be expressed in terms of degree of polymerization, essentially the number of monomer units which comprise the polymer.
POLIMERIZATION:
polymerization can be achieved by 2 methods.
1.Condensation polymerization
2.Addition polymerization

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PROPERTIES OF POLYMERS:
Morphological properties
Bulk properties
Chemical properties
mechanical properties
thermal properties
1. MORPHOLOGICAL PROPERTIES :
Molecular shape and the way molecules are arranged in a solid are important factors in determining the properties of polymers.
CRYSTALLANITY : The morphology of most polymers is semi crystalline. That is, they form mixtures of small crystals and amorphous material and melt over a range of temperature instead of at a single melting point. The crystalline material shows a high degree of order formed by folding and stacking of the polymer chains. The amorphous or glass like structure shows no long range order, and the chains are tangled.

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There are some polymers that are completely amorphous, but most are a combination with the tangled and disordered regions surrounding the crystalline areas.
The glass transition temperature is the point at which the polymer hardens into an amorphous solid. This term is used because the amorphous solid has properties similar to glass.
In the crystallization process it has been observed that relatively short chains organize themselves into crystalline structures more readily than longer molecules.
Therefore the degree of polymerization (DP) is an important factor in determining the crystallinity of a polymer. Polymers with a high DP have difficulty organizing into layers because they tend to become tangled.

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The cooling rate also influences the amount of crystalline. lower molecular weight polymers(Short chains)are generally weaker in strength. Although they are crystalline, only weak Van der Walls forces hold the lattice together. This allows the crystalline layers to slip past one another causing a break in the material. High DP(amorphous) polymers, however, have greater strength because the molecules become tangled between layers.
Also influencing the polymer morphology is the size and shape of the monomers substituent group.

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very few polymers can stretch out perfectly straight, and those are ultra high molecular weight polyethylene, and aramids like Kevlar and Nomex. Most polymers can only stretch out for a short distance before they fold back on themselves. You can see this in the picture.

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But not only do polymers fold like this. Polymers also form stacks of these folded chains. There is a picture of a stack, called a lamella. Of course, it isn't always as neat as this. Sometimes part of a chain is included in this crystal, and part of it isn't. When this happens we get the lamella is no longer neat and tidy, but sloppy, with chains hanging out of it everywhere.

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Of course, being indecisive, the polymer chains will often decide they want to come back into the lamella after wandering around outside for awhile. When this happens, we get a picture like this:

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AMORPHOUSNESS AND CRYSTALLINITY : Most crystalline polymers are not entirely crystalline. The chains, or parts of chains, that aren't in the crystals have no order to the arrangement of their chains. That is amorphous state. So a crystalline polymer really has two components: the crystalline portion and the amorphous portion.

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Why? So why is it that some polymers are highly crystalline and some are highly amorphous? There are two important factors, polymer structure and intermolecular forces.
CRYSTALLINITY AND POLYMER STRUCTURE : A polymer's structure affects crystallinity. If it's regular and orderly, it will pack into crystals easily. If it's not, it won't. It helps to look at polystyrene to understand how this works. There are two kinds of polystyrene. Atactic polystyrene, and Syndiotactic polystyrene. One is very crystalline, and one is very amorphous.

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CRYSTALLINITY AND INTERMOLECULAR FORCES :
Intermolecular forces can be a big help for a polymer to form crystals. A good example is nylon. You can see from the picture that the polar amide groups in the backbone chain of nylon(6,6) are strongly attracted to each other. They form strong hydrogen bonds. This strong binding holds crystals together.

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polysters are another example. Let's look at the polyester we call poly(ethylene terephthalate). The polar ester groups make for strong crystals. In addition, the aromatic rings like to stack together in an orderly fashion, making the crystal even stronger.

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2.BULK PROPERTIES :
These are the properties that dictate how the polymer actually behaves on a macroscopic scale.
TENSILE STRENGTH :
The tensile strength of a material quantifies how much stress the material will endure before failing.
YOUNG’S MOLULUS OF ELASTICITY :
Young's Modulus quantifies the elasticity of the polymer. It is defined, for small strains, as the ratio of rate of change of stress to strain.
TRANSPORT PROPERTIES :
Transport properties such as diffusivity relate to how rapidly molecules move through the polymer matrix. These are very important in many applications of polymers for films and membranes.

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PURE COMPONENT PHASE BEHAVIOR :
MELTING POINT : The term "Melting point " when applied to polymers suggests not a solid-liquid phase transition but a transition from a crystalline or semi-crystalline phase to a solid amorphous phase. Though abbreviated as simply "Tm", the property in question is more properly called the "crystalline melting temperature". Among synthetic polymers, crystalline melting is only discussed with regards to thermoplastic, as thermosetting polymers will decompose at high temperatures rather than melt.
BOILING POINT : The Boiling point of a polymer substance is never defined due to the fact that polymers will decompose before reaching theoretical boiling temperatures.

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GLASS TRASITION TEMPERATURE (Tg) : The Glass transition temperature (Tg) which describes the temperature at which amorphous polymers undergo a second order phase transition from a rubbery, viscous amorphous solid to a brittle, glassy amorphous solid. The glass transition temperature may be engineered by altering the degree of branching or cross-linking in the polymer or by the addition of plasticizer.
POLYMER SOLUTION BEHAVIOR : polymeric mixtures are far less miscible than mixtures of small molecule materials. polymeric molecules are much larger and hence generally have much higher specific volumes than small molecules, the number of molecules involved in a polymeric mixture are far less than the number in a small molecule mixture of equal volume.

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In a good solvent, the polymer appears swollen and occupies a large volume. In this scenario, intermolecular forces between the solvent and monomer subunits dominate over intramolecular interactions. In a bad solvent or poor solvent, intramolecular forces dominate and the chain contracts. 3.CHEMICAL PROPERTIES OF POLYMERS :
The attractive forces between polymer chains play a large part in determining a polymer's properties. Different side groups on the polymer can lend the polymer to ionic Bonding or hydrogen Bonding between its own chains. These stronger forces typically result in higher tensile strength and melting points.
The intermolecular forces in polymers can be affected by dipoles in the monomer units. Polymers containing amide or carbonyl groups can form Hydrogen Bonds between adjacent chains; the partially positively charged hydrogen atoms in N-H groups of one chain are strongly attracted to the partially negatively charged oxygen atoms in C=O groups on another.

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Polyesters have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogen atoms in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so a polyester's melting point and strength are lower than Kevlar ‘s (Twaron), but polyesters have greater flexibility.
The attractive forces between polyethylene chains arise from weak van der Waals forces. Van der Waals forces are quite weak, however, so polyethene can have a lower melting temperature compared to other polymers.
4.MECHANICAL PROPERTIES :
The cause of deformation is stress, that is the applied force F per unit area of cross section A.

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Stress in tension is called tensile stress . The stress needed to break the sample is the tensile strength of the material, tensile stress, and tensile strength as well, are both measured in units of force divided by units of area, usually N/m2. Stress and strength can also be measured in megapascals (MPa) or gigapascals (GPa).
Strength tells us is how much stress is needed to break something. Elongation is a type of deformation. Deformation is simply a change in shape that anything undergoes under stress. Strain or deformation in tension is called elongation € . It is the increasing length ▲L = L-Lo relative to the original length Lo. i.e.
€=L-Lo/Lo=▲L/Lo
In which L is the length under a given tensile stress .
▲L/Lo *100 = % Elongation ﻭ

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According to Hooks law, the stress is directly proportional to the strain. In tension
F/A = E { L-Lo/Lo}
The proportionality constant E ,called Young’s modulus or modulus of elasticity, is a measure of the hardness, stiffness or rigidity of the solid, that is of its resistance to deformation.
E = ▲ / ▲€. Elastomers need to show high elastic elongation. But for some other types of materials, like plastics, it usually better that they not stretch or deform so easily. If we want to know how well a material resists deformation, we measure modulus. To measure tensile modulus, we plot a stress-strain curve.

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There are times when the stress-strain curve isn't nice and straight, like we saw above. For some polymers, especially flexible plastics, we get odd curves that look like this: The slope isn't constant as stress increases. The slope, that is the modulus, is changing with stress.
In general, fibers have the highest tensile moduli, and elastomers have the lowest, and plastics have tensile moduli somewhere in between fibers and elastomers.

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Modulus is measured by calculating stress and dividing by elongation, and would be measured in units of stress divided by units of elongation. But since elongation is dimensionless, it has no units by which we can divide. So modulus is expressed in the same units as strength, such as N/cm2. That plot of stress versus strain can give us another very valuable piece of information that is Toughness. Toughness is really a measure of the energy a sample can absorb before it breaks. How is toughness different from strength? From a physics point of view, the answer is that strength tells how much force is needed to break a sample, and toughness tells how much energy is needed to break a sample. Take a look at the one below, the one with three plots, one blue, one red, and one pink.

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The blue plot is the stress-strain curve for a sample that is strong, but not tough. It takes a lot of force to break this sample, but not much energy.
On the other hand, the red plot is a stress-strain curve for a sample that is both strong and tough. This material is not as strong as the sample in the blue plot. It can absorb a lot more energy than the blue sample can. The red sample elongates a lot more before breaking than the blue sample does.

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Mechanical Properties of Real Polymers : It would probably be a good idea to talk about which polymers show which kinds of mechanical behavior, that is, which polymers are strong, which are tough, and so forth.
This graph compares typical stress-strain curves for different kinds of polymers.

Polymer degradation :

Polymer degradation Degradation of polymer is primarily the process of chain cleavage leading to reduction of molecular weight.
Erosion is the sum of all processes leading to loss of polymer matrix.
Degradation is either through bulk erosion and surface erosion. 43

Collagen :

Collagen Advantages of using the collagen:
Easy to isolate and purify in large quantities.
Biocompatible and non toxic profile.
Well established physiochemical, structural and immunological properties. 50

pH sensitive polymers: :

RECENT POLYMERS :

RECENT POLYMERS DENDRIMERS:
Ideal candidates among model hyperbranched polymers because of their well-defined structure and high density of functional groups.
Use of such nano-structured systems for targeted drug delivery is being explored. This may be achieved by attaching ligands or antibodies to the surface groups, and using the multivalency effect to improve targetabilty. Proposed structure of drug/dendrimer complex with targeting moiety may look as shown in figure. 68

Structure of dendrimer :

Structure of dendrimer 69

Uses of polymers :

Uses of polymers 70

AS GENE CARRIER: :

AS GENE CARRIER: 71

CONCLUSION- :

CONCLUSION- The knowledge and skill in area of biodegradable polymer technology is expanding rapidly.
This cutting edge technology has generated a substantial number of biodegradable polymers with wide range of degradation rates.
This has widened the horizon of the options that researchers have at their disposal for controlled and targeted delivery of a whole array of therapeutic moieties. 72